The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with automatic tuning of radio frequency transmitters and receivers in MR imaging systems and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in spectroscopy and other processes and apparatus in which transmitters and receivers are automatically tuned for optimum performance.
In magnetic resonance imaging, a uniform magnetic field is created through an examination region in which a subject being examined is disposed. A series of radio frequency pulses and magnetic field gradients are applied to the examination region. Typically, an RF coil disposed about the examination region is driven by a transmitter to excite magnetic resonance in dipoles disposed within the examination region. Via the same RF coil or alternately a separate receive-only coil, magnetic resonance signals generated by relaxing dipoles are received by a digital receiver. The magnetic resonance signals are then processed to generate two or three dimensional image representations of a portion of the subject in the examination region.
Prior to running each separate magnetic resonance scan, the frequency of the transmitter and receiver are set to the appropriate Larmor frequency such that the desired species is excited. Generally, water and/or fat molecules are largely responsible for the production of the magnetic resonance signals. The Larmor frequency of these two species is different, and for each the optimum frequency varies from patient to patient and/or from location to location within the same patient due to various inhomogeneities.
In the past, the frequency setting was accomplished either manually or automatically. In the case of manual setting, a magnetic resonance signal was generated and received such that an operator viewing a spectral magnitude of the signal could manually adjust the frequency of the transmitter and receiver to align with the signal peaks corresponding to either the Larmor frequency for fat, water, or other desired species or other particular frequency depending upon the scan to be run. This manual method is time consuming and the accuracy of the results depend on the training level and ability of the operator.
In previous automatic techniques, the frequency of the transmitter and receiver is set by performing a correlation between a spectral magnitude of the received magnetic resonance signal and a model spectrum. The method employed a two-stage process. In the first stage, a broad band signal is acquired and used to set a course frequency. In the second stage, a narrower band signal is employed and the spectral magnitude of the received magnetic resonance signal therefrom is compared with the model spectrum. This technique for automatically setting the frequency fails to account for the fact that the received signal is a function of the frequency of the transmitter. As the frequency is changed, the nature of the received signal can change. Further, no attempt is made to verify that the predicted frequency of the transmitter actually produces the desired change in the signal.
The present application contemplates a new and improved method for automatically setting the optimum frequency of a radio frequency transmitter and receiver in a magnetic resonance imaging apparatus which overcomes the above-referenced problems and others.